Raw natural gas can be treated by means of the methods described by documents FR-2,605,241 and FR-2,636,857. These methods use a physical solvent such as methanol for dehydration, gasoline extraction and removal of the acid compounds and of part of the mercaptans. After this treatment, the gas meets the specifications as regards the CO2 content, typically below 2% by mole, and the H2S content, typically below 4 ppm by mole.
Another gas treatment solution consists in carrying out deacidizing by means of a method using an amine solvent. After this treatment, the gas meets the specifications relative to the CO2 content, typically below 2% by mole, and the H2S content, typically below 4 ppm by mole, i.e. a total sulfur content of the order of 6 mg S/Nm3. Part of the light mercaptans, notably methylmercaptan, is removed during this stage. On the other hand, the heavier mercaptans, such as ethyl-, propyl- and butyl-mercaptan, are not sufficiently acid to significantly react with the amines and therefore remain in the gas in a large proportion. In some cases, the mercaptan content can reach 500 ppm by mole.
The gas is then dehydrated, for example by means of a method using a solvent such as glycol, for instance the method described by document FR-2,740,468. Dehydration allows the water content of the gas to be lowered to a value close to 60 ppm by mole.
Besides, a TSA (Thermal Swing Adsorption) type absorption method on a molecular sieve, for example of 3A, 4A, 5A or 13X type, or on alumina or silica gel, can be used. In this case, the water content of the gas is typically below 1 ppm by mole.
The aforementioned methods allow to obtain a natural gas whose water, acid compound and heavy hydrocarbon contents meet the commercial requirements. However, the methyl- and ethyl-mercaptans still remain predominantly in the gas, in proportions that can reach 200 ppm by mole or more in sulfur equivalent. For certain uses, these mercaptan proportions are too high.
One object of the present invention is to provide a natural gas purification method in order to obtain a mercaptan molar content below 20 ppm.
It is possible to use a removal method using mercaptan absorption. The conventional gas phase absorption methods are the methods commonly referred to as TSA (Thermal Swing Adsorption) wherein the absorption stage takes place at ambient or moderate temperature typically ranging between 20° C. and 60° C., and the desorption (or regeneration) stage at a high temperature typically ranging between 200° C. and 350° C., in a purge gas sweep stream (in general part of the purified gas containing methane and/or ethane) whose flow rate also ranges between 5% and 20% of the flow of feed gas. The desorption gas containing a large amount of mercaptans then has to be treated prior to being recycled, for example by contacting with a basic solution (soda or potash), or it can be sent to the flare, which is neither economically nor ecologically advantageous. The pressure is either kept substantially constant throughout the cycle, or lowered during the regeneration stage so as to favor regeneration. After this stage of purification by absorption, the water content of the gas is below 1 ppm by mole, and the gas meets the total sulfur specifications.
However, absorption of the mercaptans by means of a conventional TSA method used in the industry, notably to purify a natural gas, has several drawbacks. The following can be mentioned in particular:                it requires heating to high temperatures, which leads to premature aging of the absorbent material, notably during the desorption of thermally fragile products (under the effect of the thermal treatments regularly applied during the absorbent material regeneration stage, the mercaptans can form reactive compounds and react with the co-absorbed hydrocarbons so as to eventually lead to premature aging of the absorbent material, which may require frequent renewal thereof, and thus involve an additional cost),        use of a large amount of purge gas, generally ranging between 5% and 20% of the flow of gas to be treated,        it requires treating the purge gas containing the desorbed products and recycling the purge gas.        
The present invention provides a method of purifying a natural gas by mercaptan absorption, avoiding the drawbacks of the methods from the prior art.